"The Queen's grand-daughter, a professional three-day eventer who is expecting her first baby with husband Mike Tindall, has cancelled a string of gruelling trials until after the birth early next year," says the newspaper.

A spokesman for Princess Anne's daughter – who is just over three months pregnant and was photographed taking part in an event in Sussex on Sunday – stressed there was nothing medically wrong.

But she told the newspaper that the royal's condition had weighed heavily on her decision to quit, almost a year after she won silver at the London Olympics as part of the British three-day eventing team.

The spokesman added: "The fact is that all of Zara's horses have now qualified for the events she wishes to enter them for next year.

"She has done so well already this season, she does not actually need to compete any more. Given that she is pregnant, she has decided to cancel the rest of her entries for the year.

"As we said previously, she is feeling fine and is following all medical advice from her doctors. She is just being sensible about this.

"If there is no reason for her to ride, then she won't.

"And now her horses have all qualified, there is no real point in her taking part, particularly given the fact that she is pregnant."

A friend of the royal, who is 14th in line to the throne, insisted, however, that she still plans to ride privately as her horses need to be exercised.

The friend said: "It's business as usual at the yard as far as she is concerned and Zara will continue to ride.

"She also plans to be back in the saddle as soon as possible after having her baby – she is a professional sportswoman after all."